For the purpose of improving both the light power and the energy efficiency of light sources used in many fields, such as vehicle headlights or video projectors, current technical changes are leading to replacing filament lamps with high-intensity gas discharge lamps. Unlike conventional bulbs, which were designed to be connected directly to a low-voltage source, such as the battery of a vehicle, or a medium voltage, such as the domestic mains at 110 or 220 V, these new lamps require high voltages, AC or DC, according to the operating mode, in order to create and maintain the electrical discharge in the gas.
In the automotive field, these high voltages, specific to each type of lamp (around 25 kV for lighting a xenon lamp), are produced from the onboard voltage by a supply module, which also provides the regulation, known by the name “ballast”.
The most recent ballasts usually comprise a DC to DC converter with chopping producing a continuous high voltage of several hundreds of volts from the 12 V of the battery, a DC to AC converter supplying the lamp in DC regime from the DC high voltage, and a voltage step-up circuit supplying a generator producing the very high voltage necessary for igniting the lamp.
A ballast of this type is described in detail in French patent application FR2791218.
The DC to DC converter used comprises a voltage step-up transformer whose primary winding is connected in a series with a switching device at the terminals of the DC voltage source. The repeated interruption of the latter by the switching device induces, at the terminals of the secondary winding of the transformer, a high voltage rectified by a diode and filtered by a capacitor.
The DC to AC converter of the ballast comprises four switching transistors connected as an inverter bridge, the simultaneous opening or closing of one pair of opposite transistors being controlled in alternation with that of another pair in order to produce an alternating high voltage from the DC high voltage.
A control unit controls the device and the switching transistors.
The very high voltage generator of the ballast comprises an ignition transformer, the secondary winding of which is connected in series with the discharge lamp at the output terminals of the DC to AC converter. The primary winding of the transformer is connected in series with a spark generator at the terminals of a capacitor charged by the step-up circuit.
The step-up circuit of this ballast consists of a supplementary secondary winding of the transformer of the DC to DC converter supplying a rectify/filtering cell.
The use of a transformer having two secondary windings in the DC to DC converter of a ballast is a simple and well known solution that makes it possible to effectively produce the high voltage necessary to the lamp ignition voltage generator.
However, the design and manufacture of such a transformer are relatively complex since the voltages and currents involved are high.
A known model of transformer of this kind comprises a primary of 4 turns and two secondaries of 20 turns and 34 turns in series. The voltages developed in operation at the terminals of the secondary are −400 V and 600 V respectively. The primary of the transformer being connected to the low-voltage source, the problems of insulation that must be resolved can easily be conceived.
In addition, the presence of the two windings and of their connection terminals necessarily results in a high form factor.
Finally, the high number of turns on the transformer gives rise to a high distributive capacitance that is opposed to a high operating frequency of the chopping converter and therefore to optimum efficiency.
Having regard to these drawbacks, the aim of the invention is to eliminate the supplementary secondary winding of the step-up transformer of the DC to DC converter, replacing it with a step-up circuit of another type.